Phosphate rock is a major phosphate source for agricultural fertilizers. Before chemical processing to produce fertilizers, the phosphate ore is mined and beneficiated to upgrade phosphate content. Phosphate content is generally measured in terms of P.sub.2 O.sub.5. During beneficiation the phosphate content is upgraded from about 4 to 10 percent P.sub.2 O.sub.5 to about 31 to 33 percent P.sub.2 O.sub.5. Clays and silica are the major impurities found in Florida phosphate ore. In general, Florida phosphate ore deposits, i.e., matrix, contain about a third recoverable phosphate, a third silica, i.e., sand, and a third clay, i.e., -150 mesh size (Tyler) fraction materials.
In a typical Florida phosphate mining operation, the matrix is excavated by a dragline, slurried, and pumped to the beneficiation plant for processing. A typical beneficiation plant contains three major processes, sizing, desliming, and flotation. Products in the beneficiation plant are classified according to the size fractions measured in terms of mesh size (Tyler).
As the matrix is delivered to the beneficiation plant, the sizing process produces a product called pebble. Pebble is the first final product of the beneficiation process and contains phosphate particles greater than 14 mesh size. Materials containing particles smaller than 14 mesh size are reported to a desliming process for clay separation. Clay is a waste by-product of the beneficiation process and contains particles smaller than 150 mesh size. During the desliming process, most of the clay is removed from the matrix and pumped to storage areas called clay settling ponds. As the clay is settled in the clay settling ponds, process water is reclaimed and routed for reuse in the mining and beneficiation processes.
The majority of clay is rejected as the phosphate pebble is sized. The materials remaining after sizing, called feed, contain particles ranging from -14 mesh to +150 mesh sizes. This feed contains phosphate particles (4 to 10 percent of P.sub.2 O.sub.5), 70 to 80 percent silica, and a small residue of clay. In order to remove the silica from the phosphate particles, the feed is fed to a flotation process which normally contains two flotation circuits. The first flotation circuit is called a rougher flotation circuit. The second flotation circuit is called a cationic or cleaner flotation circuit.
In the rougher flotation circuit, the silica content is reduced from 70-80 percent to 20-30 percent. The feed slurry is preconditioned with a fatty acid soap. The fatty acid soap is preferentially adsorbed by the phosphate particles, causing the phosphate particles to become hydrophobic. When the hydrophobic phosphate particles are introduced into the flotation device the phosphate particles attach to the air bubbles and float. Most of the silica particles remain in the water and sink. The floated phosphate ore, i.e., rougher concentrate, is then de-oiled by scrubbing with dilute sulfuric acid (H.sub.2 SO.sub.4) at a pH of 2.5-4.0, followed by rinsing with water.
The rougher concentrate slurry is subsequently fed into the cationic or cleaner flotation circuit for a second froth flotation process, in which the silica is reduced further to 3-10 percent. In the cationic or cleaner flotation circuit the rougher concentrate is treated with reagents which act as silica collectors. Oftentimes these cationic collectors are long-chain amines, in which case the cationic or cleaner flotation circuit is referred to as an amine flotation circuit. Accordingly, all flotation circuits using amines as collectors are referred to as amine flotation circuits. As such, the feed which reports to an amine flotation circuit is called an amine feed. In the cationic or cleaner flotation circuit, the collector is preferentially adsorbed by the silica causing the surface of the silica particles to become hydrophobic. The hydrophobic silica particles attach to the air bubbles and float. As the silica is removed, the remaining materials in the cationic or cleaner circuit are called concentrate. This concentrate is the second final product of the beneficiation plant.
In some beneficiation plants, materials containing size fractions smaller than 14 mesh and greater than 28 mesh are mechanically separated (i.e., screen or hydro-separation) from the feed. These materials are called an intermediate product. For some uses, the intermediate product is the final product of a beneficiation plant and does not require any flotation. Following this separation, the remainder of the feed (-28 mesh to +150 mesh) reports to the flotation process. Flotation feed size fractions generally vary from plant to plant depending on the overall plant flowsheet.
In beneficiation processes, large amounts of process water are used and subsequently recycled for reuse in all phases of processing such as sizing, desliming, washing, rinsing, and flotation. Fresh water from aquifers is pumped through deep wells and used in conjunction with the recycled process water. This recycled water contains both clay and phosphate slimes, with the majority of the slime being clay slime.
Recently, the conservation of fresh water in aquifers has been one of the most challenging issues facing the Florida phosphate industry. As part of the fresh water conservation effort, the deep well water which was previously used in beneficiation processes has been replaced by recycled process water. In order to hasten the settling process of clay slime and to facilitate water reclamation, a great deal of research has been done on the use of polymers to flocculate clay slime in settling ponds, for example, U.S. Pat. No. 3,020,231 to Colwell et al., EP 0 455 077 A2 to von Bonin et al., JP 3-8498 (1991) to Nippon Shokubai Kagaku, U.S. Pat. No. 4,265,770 to Thomas, U.S. Pat. No. 4,241,363 to Chamberlain et al. and U.S. Pat. No. 5,104,551 to Davis et al. Settling pond clay slime flocculation methods have also used polymers in combination with other materials, for example, U.S. Pat. Nos. 4,498,993 and 4,478,736 to Raba, Jr. et al., JP 53-99656 (1978) to Agency of Ind. Sci. Tech., JP 3-161099 to Kyoritsu Yuki Kogyo and U.S. Pat. No. 3,932,275 to Mewes et al. However, because of the economics, flocculants are rarely used in the field.
Therefore, the recycled process water from these reclamation and settling processes can still contain significant amounts of (-150 mesh) slime particles. The increasing use of slime contaminated process water in beneficiation processes imposes significant processing problems, particularly in flotation processes.
Two major problems are commonly observed. One of the problems is a marked increase in reagent, also known as collectors, consumption. Collectors useful in phosphate beneficiation include anionic collectors and cationic or amine collectors. The presence of slime causes the depletion of the collectors for the phosphate and silica flotation because the collectors are preferentially adsorbed by the fine slime particles. In order to float the phosphate or silica properly, collector dosages have to be markedly increased, especially the amine/cationic collectors. If the necessary increase in anionic collectors, for example, fatty acid/fatty acid soap and fuel oil is not provided, phosphate recovery in the rougher flotation circuit is low. If the necessary increase in cationic collectors, for example, amine is not provided, the final concentrate grade of the cationic or cleaner flotation circuit is poor. The second problem encountered, when slime is present during the beneficiation process, is the formation of a stable foam. This stable foam overflows the flotation cells and launders creating operational problems.
In view of the prior art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the pertinent art how these problems could be overcome.